Cryogenic liquids are liquified gases that have a very low critical temperature (e.g., -200.degree. F. or less), such as nitrogen, natural gas or gaseous hydrocarbons. Cryogenic liquids are typically stored or transported in vessels having a double wall vacuum jacketed construction with a multi-layer foil insulation in the vacuum space between the walls. A disadvantage of this type of multi-layer insulation is that it generally has a fixed thermal resistance. Thus, when liquid is drawn from a vessel of this type, the volume of liquid drawn must be replaced by an equal volume of gas in order to maintain the pressure in the vessel. Otherwise, the pressure of the cryogenic liquid inside the chamber will decrease, causing some of the liquid to flash to gas. Flash evaporation of the liquid reduces its temperature causing the pressure in the tank to decrease. A typical method of replacing the liquid volume removed with an equal gas volume involves directing some additional liquid drawn from the vessel through an external heat exchanger. The liquid is vaporized into a larger volume of gas in the heat exchanger and then fed back into the vessel by either a pump or gravity.
Another disadvantage of existing storage vessels is that the multi-layer foil insulation is very costly to manufacture. The heat exchanger system adds to this cost. While the cost may not be prohibitive for vessels in which the cryogenic liquid is stored for long periods of time, such as cargo ships, other applications, such as vehicle refueling stations, entail a rapid dispensing and replacement of the cryogenic liquid. In these other applications, the manufacturing and operating costs of existing insulation systems cannot be justified.